Grind 2 teaspoons juniper berries, the allspice berries, and the peppercorns in a spice grinder until coarsely ground, then stir in the salt. Press the ground spices into both sides of the flank steak; set aside to marinate at least 30 minutes.
Bring beef broth and remaining juniper berries to a simmer in a small saucepan over medium heat. Simmer until the broth has reduced by 1/4, about 5 minutes. Stir in heavy cream and gin. Return to a simmer, and cook until reduced by half, about 10 minutes. Keep sauce warm over very low heat.
Preheat an outdoor grill for medium-high heat and lightly oil grate.
Brush the flank steak with olive oil on both sides. Cook on preheated grill until cooked to desired degree of doneness, about 4 minutes per side for medium. Place steak on a plate, and allow to rest for 5 minutes.
Slice the steak thinly and place onto a serving dish. Remove the juniper berries from the gin sauce and discard. Serve the sliced steak accompanied by the gin sauce.